Reading disability (RD), math disability (MD), and attention-deficit/hyperactivity disorder (ADHD) are common disorders of childhood that frequently co-occur, but the developmental etiology of this comorbidity is largely unknown. The proposed project is an extension of the Western Reserve Reading and Math Project and the International Longitudinal Twin Study of Early Reading Development, two longitudinal twin studies that have followed population-based samples of twins since preschool. The twins will complete an extensive battery of measures of reading, math, ADHD, and neuropsychological functioning at the end of ninth grade. These measures will provide important new information regarding the developmental etiology of word reading and more complex reading fluency and comprehension, the developmental stability of DSM-IV symptom dimensions and subtypes of ADHD, and the longitudinal and cross-sectional etiology of covariance between individual differences and extreme scores on measures of reading, math, and ADHD symptoms. The extensive battery of neuropsychological measures will be used to test which neuropsychological functions are uniquely associated with RD, ADHD, or MD and which are shared risk factors across two or more of these domains, and multivariate twin analyses will be conducted to test the etiology of these associations.